Carcinogenesis is caused by multiple co-operating genetic lesions leading to a progressive deregulation of cellular signaling and cell cycle restriction point control. The mutations involved result in oncogene activation or loss of tumor-suppressor gene function. However, the mechanisms by which these mutant genes co-operate in malignant cell transformation are largely unknown. Our laboratory has shown that the co-operation of oncogenic lesions involves integration of multiple signals converging on the regulation of cell cycle-dependent kinase complexes. Here we propose to investigate the molecular mechanisms by which the c-myc oncogene co-operates with activated Ras/Raf signaling. The c-myc oncogene is frequently activated in human cancer and is a potent inducer of proliferation and apoptosis. One essential step in Myc-induced proliferation is the activation of Cyclin E-dependent kinase (Cyclin E/Cdk2). Using genetic and biochemical approaches in tissue culture, we recently made the surprising finding that activation of cyclin E/Cdk2 by c-myc requires the ability of D cyclins to sequester Gl cell cycle inhibitors p27Kip1 and p21Cip1 (Ckis). We now propose to investigate whether the capability of D cyclins to sequester Ckis plays an important role in the induction of cell division by activated c-myc in an intact organism. Moreover, we plan to tea the hypothesis that sequestration of Ckis by D cyclins may play a significant role in organ development and during multi-step carcinogenesis. This work will provide insight into developing signaling pathway-based strategies for cancer therapy in humans.